Chronic obstructive pulmonary disease (COPD) is a life-threatening, debilitating disease affecting more than 2 million Americans. New surgical therapies such as lung transplantation and lung volume reduction surgery (LVRS) can improve quality of life and pulmonary function in COPD. However, questions remain as to which patients are most likely to benefit from these therapies and why surgery is effective in some patients and not in others. We will continue an observational study of respiratory mechanics in a large cohort of patients with COPD before and after lung transplantation and LVRS. Our rationale is that our coordinated program of respiratory mechanical investigation will continue to provide insights and suggest useful measures of respiratory function that will reveal mechanisms of disease. Specific aims are: 1) To measure the maximal inspiratory pressure-volume characteristic of the chest wall in patients before and after lung transplantation or LVRS to assess changes in inspiratory function of the chest wall. We will test the hypothesis that inspiratory function of the chest wall is often compromised after operation, and that chest wall restriction is an important cause of failure to improve after LVRS or single lung transplantation for emphysema. If true, this finding would lead to studies of the origins and remedies of chest wall restriction after surgery. 2) To explore the differences between lung volumes measured by multiple breath helium dilution, plethysmography, and computed tomography (CT). Accurate measurements of lung volume are essential for diagnosis, for documentation of hyperinflation, for evaluation of patients before surgery, and for assessment of the effects of LVRS and transplantation. We will test the hypothesis that, contrary to current belief, helium dilution is more accurate than plethysmography in patients with COPD. If true, this finding would change clinical practice in this important group. 3) To develop methods to measure the pressure-area characteristics of the trachea in patients undergoing bronchoscopy and to determine whether the site of expiratory flow limitation in these patients is in the lungs or trachea. Acquired tracheomalacia is reportedly common in patients with COPD, and can cause severe expiratory obstruction that may be relieved by stenting or surgical reinforcement of the airway. We will test the hypothesis that many patients with demonstrated tracheomalacia and central airway collapse have obstruction caused by collapse of intrapulmonary airways, which is not amenable to surgical correction. Our findings will provide normative data on tracheal collapsibility not currently available, explore the association between tracheomalacia and COPD, and test a method for predicting the effect of stenting on expiratory obstruction in these patients.